Image encoding device, image processing device, image display device, image encoding method, and image processing method

ABSTRACT

An image encoding device includes a dynamic range generator for outputting dynamic range data Dd 1  of block image data Dc 1 , an average value generator for outputting average value data De 1  of the block image data Dc 1 , a number-of-pixel reducing unit  20  for decreasing number of pixels of the block image data by reduction-number of pixels to generate reduced-number-of-pixel block image data Dc 1 ′, an encoding parameter generator  18  for generating encoding parameter pa 1  specifying a quantization bit rate and the reduction-number of pixels in accordance with the dynamic range data Dd 1 , a quantization threshold generator  19  for generating a quantization threshold value tb 1 , and an image data quantizer  21  for generating quantized image data Df 1  from the reduced-number-of-pixel block image data Dc 1 ′ with use of the quantization threshold value tb 1.

TECHNICAL FIELD

The present invention relates to an image encoding device, an imageprocessing device including the image encoding device, an image displaydevice including the image processing device, an image encoding method,and an image processing method, which are used for encoding input imagedata on a block-by-block basis.

BACKGROUND ART

Since a liquid crystal panel is thin in thickness and light in weight,it is widely used as a display device such as a television receiver set,a computer display, a display of a personal digital assistant or thelike. However, since a liquid crystal requires a given time after adrive voltage is applied to the liquid crystal until transmissivity ofthe liquid crystal reaches a predetermined level, the liquid crystal hasa defect that it cannot display a fast-varying motion picture of highquality. In order to solve such a problem, when a gradation value variesbetween successive frames, a driving method, in which an overvoltage isapplied to a liquid crystal so as to cause transmissivity of the liquidcrystal to reach a predetermined level within a period of one frame, isemployed (e.g., see Patent Document 1). More specifically, image data ofa frame preceding by one frame is compared with image data of thecurrent frame on a pixel-by-pixel basis. If it is decided that thegradation value varies between pixels, an amount of correctioncorresponding to an amount of change is added to the image data of thecurrent frame. As a result, when a gradation value increases as comparedwith that of the pixel of a frame preceding by one frame, a drivevoltage higher than a normal voltage is applied to the liquid crystalpanel; whereas, when a gradation value decreases, a drive voltage lowerthan a normal voltage is applied to the liquid crystal panel.

The implementation of the aforementioned method requires a frame memoryfor outputting image data of the frame preceding by one frame. In recentyears, since number of display pixels increases due to increased size ofthe liquid crystal panel, capacity of the frame memory is also requiredto be increased. Furthermore, since the increased number of displaypixels increases the amount of data written in the frame memory or readout from the frame memory in a predetermined period (e.g., in a periodof one frame), it is necessary to increase a data transmission rate byincreasing a clock frequency for controlling the writing and readingoperation. Such increased capacity of the frame memory and suchincreased data transmission rate lead to an increase in the cost of aliquid crystal display device.

In order to solve such problems, in an image processing circuit fordriving a liquid crystal described in Patent Document 2, the necessarycapacity of a frame memory is decreased by encoding image data and thenstoring the encoded image data in the frame memory. Further, the imagedata is corrected in accordance with a result of comparison betweendecoded image data of the current frame obtained by decoding the encodedimage data and decoded image data of a frame preceding by one frameobtained by delaying the encoded image data by a period of one frame andthen decoding it. As a result, when still picture data is inputted, anunwanted overvoltage caused by errors in encoding and decoding operationcan be prevented from being applied to the liquid crystal.

Patent Document 1 is Japanese Patent Publication No. 2,616,652(Paragraphs 0025-0026, FIG. 14), and

Patent Document 2 is Japanese Patent Application Kokai (Laid-Open)Publication No. 2004-163842 (Paragraphs 0021-0042, FIG. 1).

DISCLOSURE OF THE INVENTION

Problem to be Solved by the Invention

In the image processing circuit for driving a liquid crystal describedin Patent Document 2, the encoding is carried out by a block encodingmethod, in which number of pixels of quantized image data in encodedimage data is constant regardless of the format of a received image.Thus, when the amount of encoded image data is decreased by increasing acompression rate of the encoding, an error caused by the encoding anddecoding increases and it largely reflects the corrected image data.This results in a problem that an unwanted overvoltage is applied to theliquid crystal when the amount of encoded image data is decreased byincreasing the compression rate of the encoding.

The present invention has been made for solving the problems in theabove-mentioned conventional art, it is therefore an object of thepresent invention to provide an image encoding device, an imageprocessing device including the image encoding device, an image displaydevice including the image processing device, an image encoding method,and an image processing method, which can reduce an amount of encodedimage data while suppressing an encoding error.

Means of Solving the Problems

An image encoding device according to the present invention includes: animage data blocking unit which divides image data of a current frameinto a plurality of blocks to obtain block image data, therebyoutputting the block image data; a dynamic range generator which finds adynamic range of the block image data, thereby outputting dynamic rangedata indicative of the dynamic range; an average value generator whichcalculates an average value of the block image data, thereby outputtingaverage value data indicative of the average value of the block imagedata; a number-of-pixel reducing unit which receives reduction-number ofpixels and reduces number of pixels of the block image data by thereduction-number of pixels, thereby generating reduced-number-of-pixelblock image data; an encoding parameter generator which generates anencoding parameter specifying a quantization bit rate and thereduction-number of pixels in accordance with the dynamic range data; aquantization threshold generator which generates a quantizationthreshold value in accordance with the dynamic range data, the averagevalue data, and the encoding parameter; and an image data quantizerwhich quantizes the reduced-number-of-pixel block image data with use ofthe quantization threshold value, thereby generating quantized imagedata.

Further, an image processing device according to the present inventionincludes: in addition to the above-mentioned image encoding device, afirst decoder which decodes the encoded image data to obtain firstdecoded image data corresponding to the image data of the current frame,thereby outputting the first decoded image data; a delay unit whichdelays the encoded image data by a period corresponding to one frame; asecond decoder which decodes the encoded image data outputted from thedelay unit to obtain second decoded image data corresponding to imagedata of a frame preceding the current frame by one frame, therebyoutputting the second decoded image data; an amount-of-change calculatorwhich calculates an amount of change in each pixel between the firstdecoded image data and the second decoded image data; a one-framepreceding image computing unit which calculates reproduction image datacorresponding to the one-frame preceding image data with use of theamount of change and the image data of the current frame; and an imagedata correction unit which corrects a gradation value of the image dataof the current frame in accordance with the image data of the currentframe and the reproduction image data.

Furthermore, the image display device according to the present inventionincludes: the above-mentioned image processing device; and a displaywhich displays an image based on the image data outputted from the imageprocessing device.

Moreover, an image encoding method according to the present inventionincludes the steps of: dividing image data of a current frame into aplurality of blocks to obtain block image data, thereby outputting theblock image data; finding a dynamic range of the block image data,thereby outputting dynamic range data indicative of the dynamic range;calculating an average value of the block image data, thereby outputtingaverage value data indicative of the average value of the block imagedata; generating an encoding parameter specifying a quantization bitrate and the reduction-number of pixels in accordance with the dynamicrange data; reducing number of pixels of the block image data by thereduction-number of pixels, thereby generating reduced-number-of-pixelblock image data; generating a quantization threshold value inaccordance with the dynamic range data, the average value data, and theencoding parameter; and quantizing the reduced-number-of-pixel blockimage data with use of the quantization threshold value, therebygenerating quantized image data.

Further, an image processing method according to the present inventionincludes the steps of: encoding input image data of a current frame toobtain encoded image data by the above-mentioned image encoding method,thereby outputting the encoded image data; decoding the encoded imagedata to obtain first decoded image data corresponding to the image dataof the current frame, thereby outputting the first decoded image data;delaying the encoded image data by a period corresponding to one frame;decoding the delayed encoded image data to obtain second decoded imagedata corresponding to image data of a frame preceding the current frameby one frame, thereby outputting the second decoded image data;calculating an amount of change in each pixel between the first decodedimage data and the second decoded image data; calculating reproductionimage data corresponding to the one-frame preceding image data with useof the amount of change and the image data of the current frame; andcorrecting a gradation value of the image data of the current frame inaccordance with the image data of the current frame and the reproductionimage data.

Furthermore, another image processing device according to the presentinvention, which corrects image data indicative of a gradation value ofeach pixel of an image corresponding to a voltage applied to a liquidcrystal in accordance with an amount of change in the gradation value ofeach pixel and outputs the corrected image data, includes: an encoderwhich quantizes image data of a current frame on a block-by-block basisto obtain encoded image data corresponding to the image of the currentframe, thereby outputting the encoded image data; a first decoder whichdecodes the encoded image data outputted from the encoder to obtainfirst decoded image data corresponding to the image data of the currentframe, thereby outputting the first decoded image data; a delay unitwhich delays the encoded image data outputted from the encoder by aperiod corresponding to one frame; a second decoder which decodes theencoded image data outputted from the delay unit to obtain seconddecoded image data corresponding to image data of a frame preceding thecurrent frame by one frame, thereby outputting the second decoded imagedata; a first high frequency component emphasizer which emphasizes highfrequency components of the first decoded image data; a second highfrequency component emphasizer which emphasizes high frequencycomponents of the second decoded image data; an amount-of-changecalculator which calculates an amount of change in each pixel betweenimage data, high frequency component of which is emphasized by the firsthigh frequency component emphasizer, and image data, high frequencycomponent of which is emphasized by the second high frequency componentemphasizer; a one-frame preceding image computing unit which calculatesreproduction image data corresponding to the one-frame preceding imagedata with use of the amount of change and the image data of the currentframe; and a correction unit which corrects a gradation value of theimage data of the current frame in accordance with the image data of thecurrent frame and the reproduction image data; wherein the encoderincludes a number-of-pixel reducing unit which reduces number of pixelsof image data of each block in the image data of the current frame andadjusts reduction-number of pixels of the image data of the currentframe in each block in accordance with the dynamic range of the imagedata of the current frame in each block.

Effects of the Invention

According to the present invention, when image data of a current frameis quantized on a block-by-block basis to output encoded image data, thereduction-number of pixels indicative of a value, by which the number ofpixels of the quantized image data in the encoded image data is reduced,is adjusted in accordance with the dynamic range of each block.Accordingly, the present invention can advantageously reduce an amountof encoded image data while suppressing an encoding error.

Further, an image display device according to the present invention canreduce an encoding error occurring when the amount of encoded image datais reduced, and can suitably control the response speed of a displaywhile avoiding application of an unwanted overvoltage caused by theinfluence of the encoding error.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of an image displaydevice including an image processing device (i.e., an image dataprocessor) according to the first embodiment of the present invention;

FIGS. 2A to 2C are diagrams showing a response characteristic of aliquid crystal, wherein FIG. 2A shows time variation of a brightnessvalue of the current image data, FIG. 2B shows time variation of abrightness value (i.e., a value corresponding to an applied voltage tothe liquid crystal) of corrected image data, and FIG. 2C shows timevariation of display brightness of a liquid crystal panel obtained byapplying a voltage based on the corrected image data of FIG. 2B;

FIG. 3 is a block diagram showing a configuration of an image encodingdevice (i.e., an encoder shown in FIG. 1) according to the firstembodiment;

FIG. 4 is a block diagram showing a configuration of a quantizer shownin FIG. 3;

FIG. 5 is a flowchart showing operation of the encoder according to thefirst embodiment;

FIG. 6 is a block diagram showing a configuration of a decoder shown inFIG. 1;

FIG. 7 is a flowchart showing operation of the decoder shown in FIG. 6;

FIG. 8 is a flowchart showing operation of the image processing deviceaccording to the first embodiment;

FIG. 9 is a block diagram showing an exemplary configuration of an imagedata correction unit shown in FIG. 1;

FIG. 10 is a schematic diagram for explaining a configuration of alook-up table shown in FIG. 9;

FIG. 11 is a diagram showing an example of a response speed of a liquidcrystal;

FIG. 12 is a diagram showing an example of an amount of correction;

FIG. 13 is a block diagram showing another example of an image datacorrection unit;

FIG. 14 is a diagram showing an example of corrected image data;

FIG. 15 is a block diagram showing a configuration of an image displaydevice including an image processing device (i.e., an image dataprocessor) according to the second embodiment of the present invention;

FIGS. 16A, 16B1, 16B2, 16C1, and 16C2 are diagrams showing examples ofdata structure of encoded image data in the second embodiment;

FIGS. 17A, 17B1, 17B2, 17C1, and 17C2 are diagrams showing otherexamples of data structure of encoded image data in the secondembodiment;

FIG. 18 is a block diagram showing an exemplary configuration of animage processing device according to the third embodiment;

FIG. 19 is a diagram showing an internal configuration of a highfrequency component emphasizer in the third embodiment;

FIG. 20 is a diagram showing an internal configuration of anamount-of-emphasis generator in the third embodiment;

FIGS. 21A and 21B are diagrams showing influence caused by reducingnumber of pixels of encoded image data;

FIGS. 22A and 22B are diagrams showing influence caused by reducingnumber of pixels of encoded image data;

FIGS. 23A to 23D are diagrams showing operation of the high frequencycomponent emphasizer in the third embodiment;

FIGS. 24A and 24B are diagrams showing correction data used in highfrequency component emphasis process; and

FIG. 25 is a block diagram showing another exemplary configuration ofthe image processing device according to the third embodiment.

DESCRIPTION OF REFERENCE NUMERALS

1 input terminal; 2 receiver; 3 image data processor (image processingdevice); 4 encoder (image encoding device); 5 delay unit; 6 firstdecoder; 7 second decoder; 8 amount-of-change calculator; 9 one-framepreceding image computing unit; 10 image data correction unit; 11display; 12 image data blocking unit; 13 dynamic range generator; 14average value generator; 15 quantizer; 16 encoded data synthesizer; 17threshold generator; 18 encoding parameter generator; 19 quantizationthreshold generator; 20 number-of-pixel reducing unit; 21 image dataquantizer; 22 threshold generator; 23 encoding parameter determinationunit; 24 encoding data divider; 25 image data restoration valuegenerator; 26 image data restoring unit; 27 image data interpolator; 28look-up table; 29 correcting part; 30 look-up table; 40 image dataprocessor; 41, 42, 43 color space converter; 44 image data processor; 45first high frequency component emphasizer; 46 second high frequencycomponent emphasizer; 47 high frequency component detector; 48amount-of-emphasis generator; 49 amount-of-emphasis adder; 50multiplier; 51 number-of-pixel reduction determination unit; 52 imagedata processor.

BEST MODE FOR CARRYING OUT THE INVENTION

First Embodiment

FIG. 1 is a block diagram showing a configuration of an image displaydevice including an image data processor 3 which is an image processingdevice according to the first embodiment of the present invention. Asshown in FIG. 1, the image display device includes, as its mainconstituent elements, a receiver 2, an image data processor 3, and adisplay 11. In this application, constituent elements shown by “ . . .unit”, “ . . . er” or “ . . . or” may be implemented in the form ofhardware including an electric circuit or the like, in the form ofsoftware, or in the form of a combination of software and hardware.Further, the image data processor 3 is a device capable of implementingan image processing method of the present invention. Furthermore, theimage display device shown in FIG. 1 is, for example, a liquid crystaltelevision set.

The receiver 2 includes a television tuner and other elements. Thereceiver 2 receives a video signal through an input terminal 1,processes the video signal for channel selection, demodulation and so onto generate a current image data Di1 indicative of an image of one frame(i.e., an image of the current frame or a current image), andsequentially outputs it to the image data processor 3.

As shown in FIG. 1, the image data processor 3 includes an encoder 4which is an image encoding device according to the first embodiment ofthe present invention, a delay unit 5, a first decoder 6, a seconddecoder 7, an amount-of-change calculator 8, a one-frame preceding imagecomputing unit 9, and an image data correction unit 10. The image dataprocessor 3 corrects the current image data Di1 in accordance with anamount of change in the gradation value, and outputs corrected imagedata Dj1 to the display 11.

A display panel of the display 11 is, for example, a liquid crystalpanel. The display 11 changes transmissivity of each pixel in the liquidcrystal by applying a voltage corresponding to the corrected image dataDj1 indicative of an image brightness or image gradation to the liquidcrystal panel, thereby displaying an image.

Explanation will next be made as to operation of the image dataprocessor 3. The encoder 4 encodes the current image data Di1 tocompress an amount of data, thereby generating encoded image data Da1.As a coding scheme of the encoder 4, block truncation coding (BTC) suchas FBTC (Fixed Block Truncation Coding) or GBTC (Generalized BlockTruncation Coding) can be used. Further, as a coding scheme of theencoder 4, a two-dimensional discrete cosine transform coding typifiedby JPEG, a predictive coding typified by JPEG-LS, or a coding schemeusing wavelet transform typified by JPEG2000 can also be employed. Anyother coding scheme can be employed as long as it is a scheme for stillimages. Furthermore, as the coding scheme for still images, such anirreversible coding that image data before encoded does not coincidecompletely with image data after decoded, can be used. In this example,as will be described later, the encoder 4 determines number of pixels ofquantized image data in the encoded image data in accordance with thesize of the dynamic range of each block, that is, determinesreduction-number of pixels indicative of a value, by which number ofpixels of each block is reduced to obtain a reduced number of pixels,and outputs the encoded image data Da1 having the reduced number ofpixels.

The delay unit 5 delays the encoded image data Da1 generated in theencoder 4 by a period corresponding to one frame to obtain an encodedimage data Da0 of a frame preceding by one frame, and outputs theencoded image data Da0. The higher an encoding rate (i.e., a datacompression rate) of the image data Di1 in the encoder 4 is made, thesmaller the storage capacity of a memory (not shown) in the delay unit 5required for delaying the encoded image data Da1 can be made.

The first decoder 6 determines a quantization bit rate of the encodedimage data Da1 on a block-by-block basis and decodes the encoded imagedata Da1 on a block-by-block basis to obtain decoded image data Db1corresponding to the current image data Di1, and outputs the decodedimage data Db1. Further, the second decoder 7 determines a quantizationbit rate of the encoded image data Da0 delayed by the delay unit 5 by aperiod corresponding to one frame on a block-by-block basis and decodesthe encoded image data Da0 on a block-by-block basis to obtain decodedimage data Db0 indicative of image data of a frame preceding by oneframe, and outputs the decoded image data Db0.

The amount-of-change calculator 8 subtracts the first decoded image dataDb1 from the second decoded image data Db0 with use of the first decodedimage data Db1 corresponding to the current image and the second decodedimage data Db0 corresponding to the image of a frame preceding by oneframe, thereby calculating an amount of change Dv1 of the gradationvalue of each pixel between the one-frame preceding image and thecurrent image. The amount of change Dv1 and the current image data Di1are inputted to the one-frame preceding image computing unit 9.

The one-frame preceding image computing unit 9 adds the amount of changeDv1 of the gradation value outputted from the amount-of-changecalculator 8 to the current image data Di1, thereby generating one-framepreceding image data Dp0. The generated one-frame preceding image dataDp0 is inputted to the image data correction unit 10.

The image data correction unit 10 corrects the current image data Di1 inaccordance with the amount of change in the gradation value for a periodof one frame obtained by comparison between the current image data Di1and the one-frame preceding image data Dp0 in such a manner thattransmissivity of the liquid crystal reaches a predetermined levelspecified by the image data Di1 within a period of one frame, and thenoutputs the corrected image data Dj1.

FIGS. 2A to 2C are diagrams showing response characteristics when adrive voltage based on the corrected image data Dj1 is applied to theliquid crystal. FIG. 2A shows time variation of the gradation value(i.e., brightness value) of the current image data Di1, and FIG. 2Bshows time variation of the gradation value (i.e., brightness value) ofthe corrected image data Dj1. In FIG. 2C, a solid line denotes timevariation of display brightness of the liquid crystal panel (i.e., theresponse characteristic of the liquid crystal panel) obtained when thedrive voltage based on the corrected image data Dj1 is applied to theliquid crystal, and a broken line denotes a response characteristic ofthe liquid crystal panel when a drive voltage (VH or VL shown in FIG.2B) based on the corrected image data Dj1 is continuously applied to theliquid crystal. As shown in FIG. 2B, when the gradation value isincreased or decreased, the corrected image data Dj1 is generated byadding or subtracting an amount of correction V1 or V2 to or from thecurrent image data Di1 respectively. By applying the drive voltage basedon the corrected image data Dj1 to the liquid crystal, thetransmissivity of the liquid crystal can reach a predetermined levelcorresponding to the gradation value of the current image data Di1nearly within a period of approximately one frame, as shown by the solidline in FIG. 2C.

Explanation will next be made as to a configuration and operation of theencoder 4 in the image encoding device according to the firstembodiment. FIG. 3 is a block diagram schematically showing aconfiguration of the encoder 4. As shown in FIG. 3, the encoder 4includes, as its main constituent elements, an image data blocking unit12, a dynamic range generator 13, an average value generator 14, aquantizer 15, and an encoded data synthesizer 16. The image datablocking unit 12 divides the current image data Di1 into a plurality ofblocks each having a predetermined number of pixels, thereby generatingblock image data Dc1. The dynamic range generator 13 finds a dynamicrange for each block image data Dc1 outputted from the image datablocking unit 12 to output dynamic range data Dd1 indicative of thedynamic range. The average value generator 14 calculates an averagevalue of each block image data Dc1 outputted from the image datablocking unit 12 to obtain average value data De1 indicative of theaverage value, and outputs it. The quantizer 15 quantizes each blockimage data Dc1 outputted from the image data blocking unit 12 to obtainquantized image data Df1, and outputs it. The encoded data synthesizer16 combines the dynamic range data Dd1, the average value data De1, andthe quantized image data Df1 by bit combining to obtain combined data,and outputs the combined data as the encoded image data Da1.

FIG. 4 is a block diagram schematically showing a configuration of thequantizer 15. As shown in FIG. 4, the quantizer 15 includes, as its mainconstituent elements, a threshold generator 17, an encoding parametergenerator 18, a quantization threshold generator 19, a number-of-pixelreducing unit 20, and an image data quantizer 21.

The threshold generator 17 outputs a changeover threshold value ta1 tobe used when the quantization bit rate of the block image data Dc1 isswitched in accordance with the dynamic range size of the dynamic rangedata Dd1. The changeover threshold value ta1 is set, for example, at thestage of manufacturing the image processing device.

The encoding parameter generator 18 determines the quantization bit rateof the block image data Dc1 in accordance with the result of comparisonbetween the dynamic range data Dd1 and the changeover threshold valueta1. Further, the encoding parameter generator 18 determines thereduction-number of pixels of the block image data Dc1 in accordancewith the result of comparison between the dynamic range data Dd1 and thechangeover threshold value ta1. The encoding parameter generator 18outputs an encoding parameter pa1 which specifies the determinedquantization bit rate and the determined reduction-number of pixels.

The quantization threshold generator 19 calculates a quantizationthreshold value tb1 to be used when the block image data Dc1 isquantized, in accordance with the dynamic range data Dd1, the averagevalue data De1, and the quantization bit rate specified by the encodingparameter pa1. The quantization threshold value tb1 is set in accordancewith threshold value data corresponding to a value obtained bysubtracting a value of one from a value indicative of the quantizationbit rate.

The number-of-pixel reducing unit 20 reduces the number of pixels of theblock image data Dc1 by the reduction-number of pixels specified by theencoding parameter pa1, and outputs reduced-number-of-pixel block imagedata Dc1′ of pixels, number of which is not larger than the number ofpixels of the block image data Dc1. As a method for reducing the numberof pixels in the number-of-pixel reducing unit 20, various kinds ofmethods such as a simple pixel thinning method or a method of generatingan average of values of a plurality of adjacent pixels may be employedso long as the method can reduce the number of pixels.

The image data quantizer 21 quantizes each pixel data of thereduced-number-of-pixel block image data Dc1′ with use of the thresholdvalue data included in a signal of the quantization threshold value tb1to obtain the quantized image data Df1, and outputs it.

Since when the dynamic range specified by the dynamic range data Dd1 ofthe block image data Dc1 is small, an error caused by reducing thenumber of pixels is small, the encoding parameter pa1 specifies a largereduction-number of pixels. Since when the dynamic range specified bythe dynamic range data Dd1 of the block image data Dc1 is large, anerror caused by reducing the number of pixels is large, the encodingparameter pa1 specifies a small reduction-number of pixels. In this way,since the reduction-number of pixels of the block image data Dc1 isadjusted in accordance with the dynamic range, the encoding error can beminimized and the encoded image data Da1 can be made small in size.

FIG. 5 is a flowchart showing steps of encoding operations in theencoder 4. When the current image data Di1 is inputted to the image datablocking unit 12 (step St1), the image data blocking unit 12 divides thecurrent image data Di1 into a plurality of blocks to obtain the blockimage data Dc1, and outputs the block image data Dc1 (step St2). Next,the dynamic range generator 13 detects the dynamic range of the blockimage data Dc1, thereby generating the dynamic range data Dd1 (stepSt3). The average value generator 14 calculates an average value of theblock image data Dc1 to generate the average value data De1 (step St4).The encoding parameter generator 18 determines a quantization bit ratein accordance with a result of comparison between the dynamic range dataDd1 and the changeover threshold value ta1, determines reduction-numberof pixels in accordance with the dynamic range data Dd1, and outputs theencoding parameter pa1 specifying the determined quantization bit rateand the determined reduction-number of pixels (step St5). Next, thequantization threshold generator 19 calculates the quantizationthreshold value tb1 corresponding to the quantization bit rate specifiedby the encoding parameter pa1 (step St6). The number-of-pixel reducingunit 20 reduces the number of pixels of the block image data Dc1 by thereduction-number of pixels specified by the encoding parameter pa1, andoutputs the reduced-number-of-pixel block image data Dc1′ of pixels,number of which is not larger than the number of pixels of the blockimage data Dc1 (step St7). Next, the image data quantizer 21 quantizeseach pixel data of the reduced-number-of-pixel block image data Dc1′ inaccordance with the quantization threshold value tb1 to obtain thequantized image data Df1, and outputs the quantized image data Df1 (stepSt8). The encoding data synthesizer 18 combines the dynamic range dataDd1, the average value data De1, and the quantized image data Df1 by bitcombining to obtain the encoded image data Da1, and outputs the encodedimage data Da1 (step St9).

Explanation will then be made as to configurations and operation of thefirst decoders 6 and the second decoder 7. FIG. 6 is a block diagramshowing a configuration (including the same constituent elements as thesecond decoder 7) of the first decoder 6. As shown in FIG. 6, the firstdecoder 6 includes, as its main constituent elements, a thresholdgenerator 22, an encoding parameter determination unit 23, an encodingdata divider 24, an image data restoration value generator 25, an imagedata restoring unit 26, and an image data interpolator 27.

The threshold generator 22 outputs a determined threshold value tc1 setat the same value as the changeover threshold value ta1 of the encodingparameter.

The encoding parameter determination unit 23 compares a value of thedynamic range specified by the dynamic range data Dd1 included in theencoded image data Da1 with the determined threshold value tc1, findsthe encoding parameter pa1 of the encoded image data Da1, and outputsthe determined parameter as an encoding parameter pb1.

The encoding data divider 24 divides the encoded image data Da1 into thedynamic range data Dd1, the average value data De1, and the quantizedimage data Df1 by referring to the encoding parameter pb1, and outputsthe divided data.

The image data restoration value generator 25 generates restored valuedata ra1 from the dynamic range data Dd1 and the average value data De1in accordance with the encoding parameter pb1, and outputs it. Therestored value data ra1 is data made up of restored values correspondingto quantization values of the quantized image data, and the number ofthe restored values corresponds to a value of the quantization bit rate.

The image data restoring unit 26 restores reduced-number-of-pixeldecoded image data from the quantized image data Df1 in accordance withthe restored value data ra1, and outputs the reduced-number-of-pixeldecoded image data Dh1.

The image data interpolator 27 interpolates the reduced-number-of-pixeldecoded image data Dh1 having pixels, number of which is not larger thanthe number of pixels of the block image data Dc1, and outputs thedecoded image data Db1 having pixels, number of which is equal to thenumber of pixels of the block image data Dc1.

FIG. 7 is a flowchart showing steps of decoding operation in the firstdecoder 6 and the second decoder 7. When the encoded image data Da1 isinputted to the encoding parameter determination unit 23 and theencoding data divider 24 (step St11), the encoding parameterdetermination unit 23 compares the dynamic range data Dd1 included inthe encoded image data Da1 with the changeover threshold value ta1,thereby determining the encoding parameter pb1 (step St12). Next, theencoding data divider 24 divides the encoded image data Da1 into thedynamic range data Dd1, the average value data De1, and the quantizedimage data Df1 by referring to the encoding parameter pb1 (step St13).Next, the image data restoration value generator 25 generates therestored value data ra1 from the dynamic range data Dd1 and the averagevalue data De1 (step St14). Subsequently, the image data restoring unit26 restores the reduced-number-of-pixel decoded image data from thequantized image data Df1 in accordance with the restored value data ra1,and outputs the reduced-number-of-pixel decoded image data Dh1 (stepSt15). Next, the image data interpolator 27 interpolates thereduced-number-of-pixel decoded image data Dh1 having pixels, number ofwhich is smaller than the number of pixels of the block image data Dc1,and outputs the decoded image data Db1 having pixels, number of which isequal to the number of pixels of the block image data Dc1 (step St16).

FIG. 8 is a flowchart showing steps of processing in the image dataprocessor 3 as the image processing device according to the firstembodiment. When the current image data Di1 is inputted to the imagedata processor 3 (step St21), the encoder 4 encodes the current imagedata Di1 through the steps shown in FIG. 5, and outputs the encodedimage data Da1 (step St22). The delay unit 5 delays the encoded imagedata Da1 by a period of one frame, and outputs the encoded image dataDa0 of a frame preceding by one frame (step St23). The second decoder 7decodes the encoded image data Da0 of a frame preceding by one framethrough the steps shown in FIG. 7, and outputs the decoded image dataDb0 corresponding to the current image data Di0 of a frame preceding byone frame (step St24). Concurrently with operation of the steps St23 andSt24, the first decoder 6 decodes the encoded image data Da1 through thesteps shown in FIG. 7, and outputs the decoded image data Db1corresponding to the current image data Di1 of the current frame (stepSt25).

Next, the amount-of-change calculator 8 subtracts the decoded image dataDb1 from the decoded image data Db0 to find an amount of change in thegradation value of each pixel between the image of a frame preceding byone frame and the current image, and outputs a result of the subtractingas an amount of change Dv1 (step St26). Next, the one-frame precedingimage computing unit 9 adds the amount of change Dv1 to the currentimage data Di1, and outputs a result of the adding as the one-framepreceding image data Dp0 (step St27). The image data correction unit 10finds an amount of correction necessary for driving the liquid crystalin such a manner that the transmissivity of the liquid crystal reaches apredetermined level specified by the current image data Di1 within aperiod of one frame in accordance with an amount of change in thegradation value obtained by comparison between the one-frame precedingimage data Dp0 and the current image data Di1, corrects the currentimage data Di1 based on the found amount of correction, and outputs thecorrected image data Dj1 (FIG. 2B) (step St28). In this connection,processing of the above steps St21 to St28 are carried out on each ofthe pixels of the current image data Di1.

As has been described above, according to the image processing device ofthe first embodiment, when the current image data Di1 is encoded, thelarger the dynamic range of an image data of a divided block is, thesmaller the reduction-number of pixels is; whereas the smaller thedynamic range is, the larger the reduction-number of pixels is. Throughsuch a control, the amount of image data temporarily stored in the framememory of the delay unit 5 can be reduced while an encoding rategenerated in the encoder 4 can be suppressed. As a result, the capacityof the frame memory of the delay unit 5 can be made small.

In the aforementioned explanation, the image data correction unit 10 hascalculated an amount of correction in accordance with an amount ofchange in the gradation value obtained by comparison between theone-frame preceding image data Dp0 and the current image data Di1 togenerate the corrected image data Dj1. However, such an arrangement isalso possible that the amount of correction may be stored in a memoryarea such as a look-up table, so that the amount of correction is readout therefrom to correct the current image data Di1.

FIG. 9 is a block diagram showing an example of a configuration of theimage data correction unit 10. The image data correction unit 10 shownin FIG. 9 has a look-up table (LUT) 28 and a correcting part 29. Thelook-up table 28 receives the one-frame preceding image data Dp0 and thecurrent image data Di1, and outputs an amount of correction Dg1generated from values of the both data.

FIG. 10 is a schematic diagram for explaining an example of aconfiguration of the look-up table 28 shown in FIG. 9. The current imagedata Di1 and the one-frame preceding image data Dp0 are inputted to thelook-up table 28 as read addresses. When the current image data Di1 andthe one-frame preceding image data Dp0 are of eight bits respectively,(256×256) items of data are stored in the look-up table 28 as the amountof correction Dg1. The look-up table 28 reads out and outputs the amountof correctionDg1=dt(Di1,Dp0)corresponding to the values of the current image data Di1 and theone-frame preceding image data Dp0. The correcting part 29 adds theamount of correction Dg1 outputted from the look-up table 28 to thecurrent image data Di1 to obtain the corrected image data, and outputsthe corrected image data Dj1.

FIG. 11 is a diagram showing an example of a response time of the liquidcrystal. In FIG. 11, an x axis denotes a value (i.e., a gradation valuein the current image) of the current image data Di1, a y axis denotes avalue (i.e., a gradation value in the one-frame preceding image) of thecurrent image data Di0 of a frame preceding by one frame, and a z axisdenotes a response time required after the transmissivity of the liquidcrystal has a value corresponding to the gradation value of theone-frame preceding image until the transmissivity of the liquid crystalreaches a value corresponding to the gradation value of the currentimage data Di1. When the gradation value of the current image has eightbits, there are present combinations of 256 multiplied by 256 betweenthe gradation value of the current image data and the gradation value ofthe one-frame preceding image data, and therefore there are also presentcombinations of 256 multiplied by 256 between response times. In FIG.11, response times of 8 multiplied by 8 corresponding to thecombinations between the gradation values are schematically shown.

FIG. 12 is a diagram showing the amount of correction Dg1 which is addedto the current image data Di1 so that transmissivity of the liquidcrystal reaches a level specified by the current image data Di1 when aperiod of one-frame is elapsed. When the gradation value of the currentimage data has eight bits, (256×256) items of the corrected image dataDj1 between the gradation value of the current image data and thegradation value of the one-frame preceding image data are present. InFIG. 12, in a similar manner to FIG. 11, amounts of correctioncorresponding to combinations of 8 multiplied by 8 between the gradationvalues are schematically shown.

As shown in FIG. 11, since the response time of the liquid crystalvaries with the gradation values of the current image data and theone-frame preceding image data, (256×256) amounts of correction Dg1corresponding to the both gradation values of the current image data andthe one-frame preceding image data are stored in the look-up table 28.The liquid crystal has a slow response speed, in particular, in halftone(gray). Accordingly, when the amounts of correctionDg1=dt(di1,Dp0)corresponding to the one-frame preceding image data Dp0 indicative of ahalftone and to the current image data Di1 indicative of a highgradation are set at large values, the response speed can be effectivelyincreased. Further, the response characteristic of the liquid crystalvaries with the material, electrode shape, temperature and so on of theliquid crystal. Thus, when the amount of correction Dg1 corresponding tosuch use conditions are previously stored in the look-up table 28, theresponse time can be controlled in accordance with the characteristic ofthe liquid crystal.

As mentioned above, by using a look-up table 28 storing previously-foundamounts of correction Dg1, the amount of calculation when the correctedimage data Dj1 is outputted can be decreased.

FIG. 13 is a block diagram showing another configuration of the imagedata correction unit 10 in the first embodiment. A look-up table (LUT)30 shown in FIG. 13 receives the one-frame preceding image data Dp0 andthe current image data Di1, and outputs the corrected image dataDj1=(Di1,Dp0)in accordance with the values of the both data. Stored in the look-uptable 30 are (256×256) items of corrected image dataDj1=(Di1,Dp0)obtained by adding the amount of correctionDg1=(Di1,Dp0)shown in FIG. 12 to the current image data Di1. In this case, thecorrected image data Dj1 is set so as not to exceed a gradation rangedisplayable on the display 11.

FIG. 14 is a diagram showing an example of the corrected image data Dj1stored in the look-up table 30. When the gradation value of the currentimage data has eight bits, there are present (256×256) items of thecorrected image data Dj1 corresponding to combinations between thegradation value of the current image data and the gradation value of theone-frame preceding image data. In FIG. 14, only (8×8) amounts ofcorrection corresponding to combinations between the gradation valuesare illustrated for simplicity.

In this way, when previously-found corrected image data Dj1 are storedin the look-up table 30 and the corresponding corrected image data Dj1is outputted in accordance with the current image data Di1 and theone-frame preceding image data Dp0, the amount of calculation necessaryfor outputting the corrected image data Dj1 can be further reduced.

Second Embodiment

FIG. 15 is a block diagram showing a configuration of an image displaydevice including an image data processor 40 which is an image processingdevice according to the second embodiment of the present invention. InFIG. 15, constituent elements having the same as or corresponding tothose in FIG. 1 are assigned the same reference numerals. The image dataprocessor 40 of the second embodiment is different from the image dataprocessor 3 of the aforementioned first embodiment in that the imagedata processor 40 includes a color space converter 41 at the precedingstage of the encoder 4, a color space converter 42 at the subsequentstage of the first decoder 6, and a color space converter 43 at thesubsequent stage of the second decoder 7.

The color space converter 41 converts the current image data Di1 toimage data of a brightness signal Y and color signals Cb and Cr, andoutputs a converted current image data Dt1. The encoder 4 encodes thecurrent image data Dt1 and outputs the encoded image data Da1corresponding to the current image data Dt1. The delay unit 5 delays theencoded image data Da1 by a period corresponding to one frame, andoutputs an encoded image data Da0 corresponding to an image of a framepreceding the current image by one frame. The first decoder 6 and thesecond decoder 7 decode the encoded image data Da1 and Da0, and outputdecoded image data Db1 and Db0 corresponding to the current image.

The color space converters 42 and 43 convert the decoded image data Db1and Db0 of the brightness and color signals to digital signals of RGB,and output the converted image data Du1 and Du0.

The amount-of-change calculator 8 subtracts the decoded image data Du0corresponding to the image data of the current frame from the decodedimage data Du0 corresponding to the image data of a frame preceding byone frame to calculate the amount of change Dv1 of the gradation valueof each pixel between the image of a frame preceding by one frame andthe current image. The amount of change Dv1 is inputted to the one-framepreceding image computing unit 9 together with the current image dataDi1.

The one-frame preceding image computing unit 9 adds the amount of changeDv1 of the gradation value outputted from the amount-of-changecalculator 8 to the current image data Di1, thereby generating theone-frame preceding image data Dp0. The generated one-frame precedingimage data Dp0 is inputted to the image data correction unit 10.

The image data correction unit 10 corrects the image data Di1 inaccordance with an amount of change in the gradation value in a periodof one frame obtained by comparison between the current image data Di1and the one-frame preceding image data Dp0, in such a manner thattransmissivity of the liquid crystal reaches a predetermined levelspecified by the image data Di1 in a period of one frame, and thenoutputs the corrected image data Dj1.

The encoder 4 in the second embodiment, in the substantially same way asthat in the first embodiment, generates the block image data Dc1corresponding to each of the divided blocks of the current image dataDt1, and with use of the block image data Dc1, generates the quantizedimage data Df1 obtained by quantizing the dynamic range data Dd1, theaverage value data De1, and the block image data Dc1 for each of thedivided data blocks. At this time, the block image data Dc1, the dynamicrange data Dd1, the average value data De1, and the quantized image dataDf1 are each generated with respect to each of the brightness signal Yand the color signals Cb and Cr.

FIGS. 16A, 16B1, 16B2, 16C1, and 16C2 are diagrams showing examples ofstructures of encoded image data in the second embodiment. FIGS. 16A,16B1, 16B2, 16C1, and 16C2 shows examples of the dynamic range data Dd1,the average value data De1, and the quantized image data Df1, when thenumber of pixels per block included in each of the brightness signal Yand the color signals Cb and Cr is eight. In the illustrated examples, anumber given in each of rectangles denotes the number of bits in eachdata.

FIG. 16A shows the block image data Dc1 for the brightness signal Y andthe color signals Cb and Cr. FIG. 16A indicates that each block haseight pixels, each of which has 8-bit image data.

FIG. 16B1 shows the reduced-number-of-pixel block image data Dc1′ whenthe block image data Dc1 of the brightness signal Y and the colorsignals Cb and Cr has a reduced number of pixels of four. FIG. 16B2shows the reduced-number-of-pixel block image data Dc1′ when the blockimage data Dc1 of the brightness signal Y has a reduced number of pixelsof zero and when the block image data Dc1 of the color signals Cb and Crhas a reduced number of pixels of six.

FIG. 16C1 shows the encoded image data Da1 obtained when thereduced-number-of-pixel block image data Dc1′ shown in FIG. 16B1 isencoded. FIG. 16C2 shows the encoded image data Da1 obtained when thereduced-number-of-pixel block image data Dc1′ shown in FIG. 16B2 isencoded. The encoded image data of FIGS. 16C1 and 16C2 have each thedynamic range data Dd1, the average value data De1, and the quantizedimage data Df1.

In the second embodiment, when data in one block has a small dynamicrange, an error generated by decreasing the number of pixels is small.Thus, as shown in FIG. 16B2, the reduction-number of pixels for each ofthe color signals Cb and Cr is made large and the reduction-number ofpixels for the brightness signal Y is made small. Conversely, when thecolor signals Cb and Cr have each a large dynamic range, as shown inFIG. 16B1, the brightness signal Y and the color signals Cb and Cr areset to have an identical reduction-number of pixels in the secondembodiment. In this way, when the reduction-number of pixels for each ofthe brightness signal Y and the color signals Cb and Cr is adjusted inaccordance with the dynamic range of the color signals Cb and Cr, theinfluence of an error by decreasing the number of pixels can beminimized while keeping the amount of the encoded image data Da1constant.

As a method of reducing the number of pixels in the number-of-pixelreducing unit 20, any method such as a simple pixel thinning method or amethod for outputting an average of values of a plurality of adjacentpixels can be employed so long as the method can reduce the number ofpixels.

FIGS. 17A, 17B1, 17B2, 17C1, and 17C2 are diagrams showing anotherexample of structures of encoded image data in the second embodiment.FIGS. 17A, 17B1, 17B2, 17C1, and 17C2 are diagrams showing otherexamples of the dynamic range data Dd1, the average value data De1, andthe quantized image data Df1 in the second embodiment, when the numberof pixels included in the brightness signal Y per block is eight andwhen the number of pixels included in each of the color signals Cb andCr is sixteen. In the illustrated example, a number given in each ofrectangles and squares denotes the number of bits in each data.

FIG. 17A shows the block image data Dc1 when the brightness signal Y hasimage data corresponding to two blocks and when each of the colorsignals Cb and Cr has image data corresponding to one block.

FIG. 17B1 shows the reduced-number-of-pixel block image data Dc1′ whenthe brightness signal Y has reduction-number of pixels of four and wheneach of the color signals Cb and Cr has reduction-number of pixels oftwelve. FIG. 17B2 shows the reduced-number-of-pixel block image dataDc1′ when the brightness signal Y has reduction-number of pixels of zeroand each of the color signals Cb and Cr has reduction-number of pixelsof sixteen.

FIG. 17C1 shows the encoded image data Da1 obtained when thereduced-number-of-pixel block image data Dc1′ shown in FIG. 17B1 isencoded. FIG. 17C2 shows the encoded image data Da1 obtained when thereduced-number-of-pixel block image data Dc1′ shown in FIG. 17B2 isencoded. Each of the encoded image data of FIGS. 17C1 and 17C2 includesthe dynamic range data Dd1, the average value data De1, and thequantized image data Df1.

In the examples of FIGS. 17A, 17B1, 17B2, 17C1, and 17C2, the brightnesssignal Y and the color signals Cb and Cr have different numbers ofpixels included in one block, and the number of pixels corresponding totwo blocks for the brightness signal Y is equal to the number, of pixelsincluded in one block for the color signals Cb and Cr.

When data in one block has a small dynamic range, an error generated bydecreasing the number of pixels is small. In the example of FIGS. 17A to17C2, therefore, when the color signals Cb and Cr have both a smalldynamic range, the number-of-pixel reducing unit 20 increases thereduction-number of pixels until the number of pixels for the colorsignals Cb and Cr becomes zero, and the reduction-number of pixels ofthe brightness signal Y becomes zero as shown in FIG. 17B2.

Conversely, when any one of the color signals Cb and Cr has a largedynamic range, the reduction-number of pixels of the brightness signal Yis set at four and the reduction-number of pixels of the color signalsCb and Cr is set at twelve as shown in FIG. 17B1. In this way, thereduction-number of pixels is adjusted not only with use of a pair ofblock image data of the brightness signal Y and the color signals Cb andCr as in the example of FIGS. 16A to 16C2, but the reduction-number ofpixels may also be adjusted with use of block image data of a pluralityof blocks. That is, any combination between the reduction-number ofpixels can be employed so long as the combination can provide a constantamount for the encoded image data Da1.

When the reduction-number of pixels is set to be equal to the number ofpixels included in one block as in the example of FIGS. 17A to 17C2, thenumber of pixels of the reduced-number-of-pixel block image data Dc1′can be set at zero and the encoded image data Da1 can have the dynamicrange data Dd1 and the average value data De1 alone.

According to the image processing device of the second embodimentdescribed above, when the dynamic range of the color signals Cb and Cris small, the reduction-number of pixels of the brightness signal Y canbe controllably reduced simultaneously with the increasedreduction-number of pixels of the color signals Cb and Cr. As a result,an encoding error generated by reducing the number of pixels can bereduced and the amount of encoded image data can be kept constant.

Further, when the dynamic range of the color signals Cb and Cr is small,the reduction-number of pixels of the brightness signal Y iscontrollably reduced simultaneously with the increased reduction-numberof pixels of the color signals Cb and Cr and therefore an encoding errorwhen the number of pixels is reduced is made to be small. Thus, evenwhen a compression rate is increased, the corrected image data Dj1 canbe created with a small error. In other words, even when image data isreduced by encoding, the response time of the liquid crystal can besuitably controlled while avoiding application of an unwantedovervoltage caused by the encoding error, thus enabling reduction of thecapacity of the frame memory of the delay unit 5 necessary for delayingthe encoded image data Da1.

Third Embodiment

FIG. 18 is a block diagram showing a configuration of a liquid crystaldisplay device including an image data processor 44 which is an imageprocessing device according to the third embodiment of the presentinvention. In FIG. 18, constituent elements having the same as orcorresponding to those in FIG. 1 are assigned the same referencenumerals. The image data processor 44 of the image data processor 3 isdifferent from the image data processor 3 of the first embodiment shownin FIG. 1, in that a first high frequency component emphasizer 45 isprovided at the subsequent stage of the first decoder 6, a second highfrequency component emphasizer 46 is provided at the subsequent stage ofthe second decoder 7, the decoded image data Db1 and the encodingparameter pb1 are inputted to the first high frequency componentemphasizer 45 from the first decoder 6, the decoded image data Db0 andthe encoding parameter Pb0 are outputted from the second decoder 7 tothe second high frequency component emphasizer 46, and theamount-of-change calculator 8 calculates a difference between an outputDb1 a of the first high frequency component emphasizer 45 and an outputDb0 a of the second high frequency component emphasizer 46.

FIG. 19 is a block diagram showing an internal configuration of thefirst high frequency component emphasizer 45 (or the second highfrequency component emphasizer 46). In the third embodiment, the firsthigh frequency component emphasizer 45 and the second high frequencycomponent emphasizer 46 have the same constituent elements and function.As shown in FIG. 19, the first high frequency component emphasizer 45(or the second high frequency component emphasizer 46) has a highfrequency component detector 47, an amount-of-emphasis generator 48, andan amount-of-emphasis adder 49.

The high frequency component detector 47, which has a band pass filter(BPF) or the like, extracts high frequency components included in thefirst decoded image data Db1 (or the second decoded image data Db0), andoutputs a high frequency component signal R1 (or R0).

The amount-of-emphasis generator 48 outputs an emphasis signal SH1 (orSH0) based on the high frequency component signal R1 (or R0), apredetermined gain G, and the encoding parameter Pb1 (or Pb0) outputtedfrom the first decoder 6 (or the second decoder 7). FIG. 20 is a blockdiagram showing an internal configuration of the amount-of-emphasisgenerator 48. As shown in FIG. 20, the amount-of-emphasis generator 48has a multiplier 50 for multiplying the high frequency component signalR1 (or R0) by the predetermined gain G and outputting the multipliedresult as a high frequency component signal R1G (or R0G), and anumber-of-pixel reduction determination unit 51 for receiving the highfrequency component signal R1G (or R0G) from the multiplier 50 and theencoding parameter Pb1 (or Pb0). The encoder 4 of the number-of-pixelreduction determination unit 51 determines whether or not the number ofpixels of the image data is reduced in accordance with the encodingparameter Pb1 (or Pb0). When determining that the number of pixels isreduced, the number-of-pixel reduction determination unit 51 inverts thehigh frequency component signal R1G (or R0G), multiplies it by anarbitrary gain coefficient to obtain the emphasis signal SH1 (or SH0),and outputs the emphasis signal SH1 (or SH0). When determining that thenumber of pixels is not reduced, the number-of-pixel reductiondetermination unit 51 outputs “0” as the emphasis signal SH1 (or SH0).

The amount-of-emphasis adder 49 adds the emphasis signal SH1 (or SH0)outputted from the amount-of-emphasis generator 48 to the first decodedimage data Db1 (or the second decoded image data Db0), and outputs thefirst decoded image data Db1 a (or the second decoded image data Db0 a),high frequency components of which is emphasized. Other elements otherthan the aforementioned constituent elements in the elements of FIG. 18have the substantially same configuration and function as those of thecorresponding constituent elements already described in the firstembodiment.

Explanation will next be made as to operation of the image dataprocessor 44 shown in FIG. 18. FIGS. 21A and 21B are diagrams showingdata after decoded of the image data processor 44, wherein FIG. 21Ashows a case where the number of pixels is not reduced in the encoder 4,and FIG. 21B shows a case where the number of pixels is reduced in theencoder 4. In FIGS. 21A and 21B, a vertical axis denotes a brightnessvalue of a pixel and a horizontal axis denotes time. FIGS. 21A and 21Bshow cases where an image is shifted by one pixel for a period of oneframe. There are some methods for decreasing the number of pixels suchas a pixel thinning method for reducing the number of pixels by removingpixels and a method for reducing the number of pixels by replacing aplurality of adjacent pixels by a pixel having data obtained throughaveraging operation of the plurality of pixels. FIG. 21B shows a casewhere the number of pixels is reduced by the averaging operation, inwhich a new halftone pixel is generated at an edge part havingbrightness initially abruptly changed (as shown in FIG. 21A) through theaveraging operation. Such a complemented edge having the halftone ismoved, an amount of change shown in FIG. 21B is observed when a pixel iscompared between frames.

FIGS. 22A and 22B are diagrams showing a problem possibly generated whenthe high frequency component emphasizing function of the first highfrequency component emphasizer 45 and the second high frequencycomponent emphasizer 46 is disabled in the third embodiment. FIGS. 22Aand 22B show the corrected image data Dj1 outputted from the image dataprocessor 44, wherein FIG. 22A shows a case where the number of pixelsis not reduced in the encoder 4 and FIG. 22B shows a case where thenumber of pixels is reduced in the encoder 4. FIGS. 22A and 22Bcorrespond to FIGS. 21A and 21B respectively. FIG. 22A shows dataobtained, when the brightness value is increased than that of theprevious frame in comparison between consecutive two frames in FIG. 21A,by increasing the brightness value by a value corresponding to theincrease; whereas, when the brightness value is decreased than that ofthe previous frame in comparison between consecutive two frames in FIG.21A, by decreasing the brightness value by a value corresponding to thedecrease. FIG. 22B shows data obtained, when the brightness value isincreased than that of the previous frame in comparison betweenconsecutive two frames in FIG. 21B, by increasing the brightness valueby a value corresponding to the increase; whereas, when the brightnessvalue is decreased than that of the previous frame in comparison betweenconsecutive two frames in FIG. 21B, by decreasing the brightness valueby a value corresponding to the decrease. As shown in FIG. 22A, when thenumber of pixels is not reduced in the encoder 4, the amplitudes of theamounts of correction V1 and V2 added to the corrected image data arelarge. As shown in FIG. 22B, however, when the number of pixels isreduced in the encoder 4, the amplitudes of the amounts of correction V1a and V2 a are small and the brightness value is varied throughout along time (i.e., the brightness value in FIG. 22B is varied in a widerange). For this reason, only when the number of pixels is reduced, theeffect of improving the response time of the liquid crystal may bepossibly reduced.

FIGS. 23A to 23D are diagrams showing operation of the first highfrequency component emphasizer 45 and the second high frequencycomponent emphasizer 46. For the simplicity of explanation, FIG. 23Ashows, as an example, an amount of change in the signal when the decodedimage data Db1 (or Db0) shown in FIG. 21B is shifted by one pixel foreach frame. FIG. 23B shows the high frequency component signal R1 (orR0) as an output signal after the decoded image data Db1 shown in FIG.23A is processed (quadratic differential operation) by an arbitrary bandpass filter (BPF) in the high frequency component detector 47. Assumingthat Y(n) denotes a brightness value at the position of the n-th pixel,then the output of the BPF is expressed, for example, as2Y(n)−{Y(n−1)+Y(n+1)}.

FIG. 23C is obtained by inverting FIG. 23B and multiplying the invertedresult by a coefficient. FIG. 23D shows the first decoded image data Db1a (or the second decoded image data Db0 a) generated by adding theemphasis signal SH1 (or SH0) generated by sign-inverting the highfrequency component signal R1 (or R0) in the amount-of-emphasisgenerator 48, to the first decoded image data Db1 (or the second decodedimage data Db0) in the amount-of-emphasis adder 49.

FIGS. 24A and 24B are diagrams showing the corrected image data Dj1outputted from the image data processor 44 when the high frequencycomponent emphasizing function of the first high frequency componentemphasizer 45 and the second high frequency component emphasizer 46 isenabled in the image data processor 3, wherein FIG. 24A show a casewhere the number of pixels is not reduced in the encoder 4 and FIG. 24Bshow a case where the number of pixels is reduced in the encoder 4. FIG.24A and FIG. 24B correspond to FIG. 21A and FIG. 23D respectively. FIG.24A is obtained, when the brightness value is increased than that of theprevious frame in comparison between consecutive two frames in FIG. 21A,by increasing the brightness value by a value corresponding to theincrease; whereas, when the brightness value is decreased than that ofthe previous frame in comparison between consecutive two frames in FIG.21A, by decreasing the brightness value by a value corresponding to thedecrease. FIG. 24B is obtained, when the brightness value is increasedthan that of the previous frame in comparison between consecutive twoframes in FIG. 23D, by increasing the brightness value by a valuecorresponding to the increase; whereas, when the brightness value isdecreased than that of the previous frame in comparison betweenconsecutive two frames in FIG. 23D, by decreasing the brightness valueby a value corresponding to the decrease. As shown in FIG. 24B, evenwhen the number of pixels is reduced in the encoder 4, the amplitudes ofthe amounts of correction V1 b and V2 b are large. Thus, even when thenumber of pixels is reduced, the effect of improving the response timeof the liquid crystal can be sufficiently obtained.

When the number of increased or decreased pixels varies from pixelposition to pixel position, the emphasizing operation of high frequencycomponents in the entire display area causes the different amount ofcorrection to be generated with different signals or different pixelpositions, which possibly leads to degradation of a picture quality suchas flickering display screen. In the image processing device of thethird embodiment, however, the emphasis signal SH1 (or SH0) iscontrolled by the number-of-pixel reduction determination unit 51 inaccordance with the encoding parameter Pb1 (or Pb0) outputted from thefirst decoder 6 (or the second decoder 7). Thus, high frequencycomponents are emphasized when the number of pixels is reduced and thedecoded image data Db1 (or Db0) is outputted as it is as data Db1 a (orDb0 a) when the number of pixels is not reduced. As a result, even whenthe number of decreased pixels varies from pixel position to pixelposition, a constant amount of correction can be provided uniformlythroughout the entire display.

According to the image processing device of the third embodimentmentioned above, high frequency components which are reduced when thenumber of pixels is reduced and then encoded are emphasized after thedecoding. Therefore, even when the decreased number of pixels causes anincreased compression rate, the corrected image data Dj1 having lesserror can be generated even for a signal in a high frequency area. Inother words, even when the number of pixels is decreased, a sufficientovervoltage can be applied to the high frequency area of an image.

The contents already described above can hold true even when an imagedata processor 52 includes the color space converter 41 provided at theprevious stage of the encoder 4 and the color space converters 42 and 43provided at the subsequent stage of the first and second high frequencycomponent emphasizers 45 and 46, as shown in FIG. 25. In FIG. 25,constituent elements that are the same as or corresponding to those inFIG. 15 are denoted by the same reference numerals. The image dataprocessor 40 of the third embodiment is effective when the number ofdecreased pixels varies with the different brightness signals anddifferent color differential signals as shown in the above secondembodiment.

1. An image processing device comprising: an encoder which encodes input image data of a current frame to obtain encoded image data and outputs the encoded image data, the encoder comprising: an image data blocking unit which divides image data of a current frame into a plurality of blocks to obtain block image data, thereby outputting the block image data; a dynamic range generator which finds a dynamic range of the block image data, thereby outputting dynamic range data indicative of the dynamic range; an average value generator which calculates an average value of the block image data, thereby outputting average value data indicative of the average value of the block image data; a number-of-pixel reducing unit which receives reduction-number of pixels and reduces number of pixels of the block image data by the reduction-number of pixels, thereby generating reduced-number-of-pixel block image data; an encoding parameter generator which generates an encoding parameter specifying a quantization bit rate and the reduction-number of pixels in accordance with the dynamic range data; a quantization threshold generator which generates a quantization threshold value in accordance with the dynamic range data, the average value data, and the encoding parameter; an image data quantizer which quantizes the reduced-number-of-pixel block image data with use of the quantization threshold value, thereby generating quantized image data for displaying an image on a display based on the quantized image data; and an encoding data synthesizer which combines the dynamic range data, the average value data, and the quantized image data to obtain encoded image data, thereby outputting the encoded image data; a first decoder which decodes the encoded image data to obtain first decoded image data corresponding to the image data of the current frame, thereby outputting the first decoded image data; a delay unit which delays the encoded image data by a period corresponding to one frame; a second decoder which decodes the encoded image data outputted from the delay unit to obtain second decoded image data corresponding to image data of a frame preceding the current frame by one frame, thereby outputting the second decoded image data; an amount-of-change calculator which calculates an amount of change in each pixel between the first decoded image data and the second decoded image data; a one-frame preceding image computing unit which calculates reproduction image data corresponding to the one-frame preceding image data with use of the amount of change and the image data of the current frame; an image data correction unit which corrects a gradation value of the image data of the current frame in accordance with the image data of the current frame and the reproduction image data; a first color space converter which converts the input image data of the current frame into image data of a brightness signal and image data of a color signal, thereby outputting the image data of the brightness signal and the image data of the color signal to the encoder; a second color space converter which converts the image data of the brightness signal corresponding to the current frame and the image data of the color signal corresponding to the current frame, both of which are outputted from the first decoder, into image data corresponding to the current frame, thereby outputting the converted image data of the current frame to the amount-of-change calculator; and a third color space converter which converts the image data of the brightness signal corresponding to the frame preceding the current frame by one frame and the image data of the color signal corresponding to the frame preceding the current frame by one frame, both of which are outputted from the second decoder, into image data corresponding to the frame preceding the current frame by one frame, thereby outputting the converted image data corresponding to the frame preceding the current frame by one frame to the amount-of-change calculator, wherein the encoding parameter generator in the encoder generates reduction-number of pixels in such a manner that the encoding parameter generator increases the reduction-number of pixels for the color signal of each block in the image data of the current frame with decrease of the dynamic range of the image data of the color signal of each block in the image data of the current frame.
 2. An image processing device comprising: an encoder which encodes input image data of a current frame to obtain encoded image data and outputs the encoded image data, the encoder comprising: an image data blocking unit which divides image data of a current frame into a plurality of blocks to obtain block image data, thereby outputting the block image data; a dynamic range generator which finds a dynamic range of the block image data, thereby outputting dynamic range data indicative of the dynamic range; an average value generator which calculates an average value of the block image data, thereby outputting average value data indicative of the average value of the block image data; a number-of-pixel reducing unit which receives reduction-number of pixels and reduces number of pixels of the block image data by the reduction-number of pixels, thereby generating reduced-number-of-pixel block image data; an encoding parameter generator which generates an encoding parameter specifying a quantization bit rate and the reduction-number of pixels in accordance with the dynamic range data; a quantization threshold generator which generates a quantization threshold value in accordance with the dynamic range data, the average value data, and the encoding parameter; an image data quantizer which quantizes the reduced-number-of-pixel block image data with use of the quantization threshold value, thereby generating quantized image data for displaying an image on a display based on the quantized image data; and an encoding data synthesizer which combines the dynamic range data, the average value data, and the quantized image data to obtain encoded image data, thereby outputting the encoded image data; a first decoder which decodes the encoded image data to obtain first decoded image data corresponding to the image data of the current frame, thereby outputting the first decoded image data; a delay unit which delays the encoded image data by a period corresponding to one frame; a second decoder which decodes the encoded image data outputted from the delay unit to obtain second decoded image data corresponding to image data of a frame preceding the current frame by one frame, thereby outputting the second decoded image data; an amount-of-change calculator which calculates an amount of change in each pixel between the first decoded image data and the second decoded image data; a one-frame preceding image computing unit which calculates reproduction image data corresponding to the one-frame preceding image data with use of the amount of change and the image data of the current frame; an image data correction unit which corrects a gradation value of the image data of the current frame in accordance with the image data of the current frame and the reproduction image data; a first color space converter which converts the input image data of the current frame into image data of a brightness signal and image data of a color signal, thereby outputting the image data of the brightness signal and the image data of the color signal to the encoder; a second color space converter which converts the image data of the brightness signal corresponding to the current frame and the image data of the color signal corresponding to the current frame, both of which are outputted from the first decoder, into image data corresponding to the current frame, thereby outputting the converted image data of the current frame to the amount-of-change calculator; and a third color space converter which converts the image data of the brightness signal corresponding to the frame preceding the current frame by one frame and the image data of the color signal corresponding to the frame preceding the current frame by one frame, both of which are outputted from the second decoder, into image data corresponding to the frame preceding the current frame by one frame, thereby outputting the converted image data corresponding to the frame preceding the current frame by one frame to the amount-of-change calculator, wherein the encoding parameter generator in the encoder generates reduction-number of pixels in such a manner that the encoding parameter generator increases the reduction-number of pixels for the brightness signal of each block in the image data of the current frame with increase of the dynamic range of the image data of the color signal of each block in the image data of the current frame.
 3. An image processing method comprising the steps of: converting the input image data of the current frame into image data of a brightness signal and image data of a color signal; encoding the image data of the brightness signal and the image data of the color signal by the following image encoding method, thereby outputting encoded image data; dividing image data of a current frame into a plurality of blocks to obtain block image data, thereby outputting the block image data; finding a dynamic range of the block image data, thereby outputting dynamic range data indicative of the dynamic range; calculating an average value of the block image data, thereby outputting average value data indicative of the average value of the block image data; generating an encoding parameter specifying a quantization bit rate and reduction-number of pixels in accordance with the dynamic range data; reducing number of pixels of the block image data by the reduction-number of pixels, thereby generating reduced-number-of-pixel block image data; generating a quantization threshold value in accordance with the dynamic range data, the average value data, and the encoding parameter; quantizing the reduced-number-of-pixel block image data with use of the quantization threshold value, thereby generating quantized image data; and displaying, by utilizing a display, an image based on the quantized image data; wherein the reducing of the number of pixels includes any of processing of pixel thinning and processing of regarding an average value of values of a plurality of adjacent pixels as data of one pixel; decoding the encoded image data to obtain first decoded image data corresponding to the image data of the brightness signal and to the image data of the color signal, thereby outputting the first decoded image data; converting the image data of the brightness signal and the image data of the color signal as the first decoded image data into image data corresponding to the current frame; delaying the encoded image data by a period corresponding to one frame; decoding the delayed encoded image data to obtain second decoded image data corresponding to image data of a frame preceding the current frame by one frame, thereby outputting the second decoded image data; and converting the image data of the brightness signal and the image data of the color signal as the second decoded image data into image data corresponding to a frame preceding the current frame by one frame; and generating reduction-number of pixels in such a manner that the reduction-number of pixels for the color signal of each block in the image data of the current frame is increased, with decrease of the dynamic range of the image data of the color signal of each block in the image data of the current frame.
 4. An image processing method comprising the steps of: converting the input image data of the current frame into image data of a brightness signal and image data of a color signal; encoding the image data of the brightness signal and the image data of the color signal by the following image encoding method, thereby outputting encoded image data; dividing image data of a current frame into a plurality of blocks to obtain block image data, thereby outputting the block image data; finding a dynamic range of the block image data, thereby outputting dynamic range data indicative of the dynamic range; calculating an average value of the block image data, thereby outputting average value data indicative of the average value of the block image data; generating an encoding parameter specifying a quantization bit rate and reduction-number of pixels in accordance with the dynamic range data; reducing number of pixels of the block image data by the reduction-number of pixels, thereby generating reduced-number-of-pixel block image data; generating a quantization threshold value in accordance with the dynamic range data, the average value data, and the encoding parameter; quantizing the reduced-number-of-pixel block image data with use of the quantization threshold value, thereby generating quantized image data; and displaying, by utilizing a display, an image based on the quantized image data; wherein the reducing of the number of pixels includes any of processing of pixel thinning and processing of regarding an average value of values of a plurality of adjacent pixels as data of one pixel; decoding the encoded image data to obtain first decoded image data corresponding to the image data of the brightness signal and to the image data of the color signal, thereby outputting the first decoded image data; converting the image data of the brightness signal and the image data of the color signal as the first decoded image data into image data corresponding to the current frame; delaying the encoded image data by a period corresponding to one frame; decoding the delayed encoded image data to obtain second decoded image data corresponding to image data of a frame preceding the current frame by one frame, thereby outputting the second decoded image data; and converting the image data of the brightness signal and the image data of the color signal as the second decoded image data into image data corresponding to a frame preceding the current frame by one frame; and generating reduction-number of pixels in such a manner that the reduction-number of pixels for the brightness signal of each block in the image data of the current frame is increased, with increase of the dynamic range of the image data of the color signal of each block in the image data of the current frame.
 5. An image processing device which corrects image data indicative of a gradation value of each pixel of an image corresponding to a voltage applied to a liquid crystal in accordance with an amount of change in the gradation value of each pixel and outputs the corrected image data, comprising: an encoder which quantizes image data of a current frame on a block-by-block basis to obtain encoded image data corresponding to the image of the current frame, thereby outputting the encoded image data; a first decoder which decodes the encoded image data outputted from the encoder to obtain first decoded image data corresponding to the image data of the current frame, thereby outputting the first decoded image data; a delay unit which delays the encoded image data outputted from the encoder by a period corresponding to one frame; a second decoder which decodes the encoded image data outputted from the delay unit to obtain second decoded image data corresponding to image data of a frame preceding the current frame by one frame, thereby outputting the second decoded image data; a first high frequency component emphasizer which emphasizes high frequency components of the first decoded image data; a second high frequency component emphasizer which emphasizes high frequency components of the second decoded image data; an amount-of-change calculator which calculates an amount of change in each pixel between image data, high frequency component of which is emphasized by the first high frequency component emphasizer, and image data, high frequency component of which is emphasized by the second high frequency component emphasizer; a one-frame preceding image computing unit which calculates reproduction image data corresponding to the one-frame preceding image data with use of the amount of change and the image data of the current frame; and a correction unit which corrects a gradation value of the image data of the current frame in accordance with the image data of the current frame and the reproduction image data; wherein the encoder includes a number-of-pixel reducing unit which reduces number of pixels of image data of each block in the image data of the current frame and adjusts reduction-number of pixels of the image data of the current frame in each block in accordance with the dynamic range of the image data of the current frame in each block for displaying an image on a display based on the dynamic range of the image data.
 6. The image processing device according to claim 5, wherein the first high frequency component emphasizer and the second high frequency component emphasizer emphasize high frequency components when the number of pixels of the image data is reduced in the encoder.
 7. The image processing device according to claim 5, wherein correction of the gradation value by the correction unit is carried out so that, upon comparison between the image data of the current frame and the reproduction image data, when the brightness value of the current frame is increased to be higher than the brightness value of the reproduction image data, the brightness value of the current frame is increased by a value corresponding to the amount of change having a positive value, and when the brightness value of the current frame is decreased to be lower than the brightness value of the reproduction image data, the brightness value of the current frame is decreased by a value corresponding to the amount of change having a negative value.
 8. An image processing method for correcting image data indicative of a gradation value of each pixel of an image corresponding to a voltage applied to a liquid crystal in accordance with an amount of change in the gradation value of the each pixel and outputting the corrected image data, comprising the steps of: quantizing image data of a current frame on a block-by-block basis to output encoded image data corresponding to the image of the current frame; decoding the encoded image data to output first decoded image data corresponding to the image data of the current frame; delaying the encoded image data by a period corresponding to one frame; decoding the delayed encoded image data to output second decoded image data corresponding to image data of a frame preceding the current frame by one frame; emphasizing high frequency components of the first decoded image data; emphasizing high frequency components of the second decoded image data; finding an amount of change in each pixel between the first decoded image data, high frequency components of which is emphasized, and the second decoded image data, high frequency components of which is emphasized; calculating reproduction image data corresponding to the one-frame preceding image data with use of the amount of change and the image data of the current frame; correcting the gradation value of the image data of the current frame in accordance with the image data of the current frame and the reproduction image data; reducing number of pixels of the image data of each block in the image data of the current frame, wherein the number of pixels of the image data of the current frame in each block. wherein reduction-number of pixels of the image data of the current frame in each block is adjusted in accordance with the dynamic range of the image data of the current frame in each block; and displaying, by utilizing a display, an image based on the dynamic range of the image data.
 9. The image processing method according to claim 8, wherein the step of emphasizing high frequency components of the first decoded image data and the step of emphasizing high frequency components of the second decoded image data are carried out when the number of pixels of the image data is decreased.
 10. The image processing method according to claim 8, wherein the step of correcting the gradation value is carried out so that, upon comparison between the image data of the current frame and the reproduction image data, when the brightness value of the current frame is increased to be higher than the brightness value of the reproduction image data, the brightness value of the current frame is increased by a value corresponding to the amount of change having a positive value, and when the brightness value of the current frame is decreased to be lower than the brightness value of the reproduction image data, the brightness value of the current frame is decreased by a value corresponding to the amount of change having a negative value. 